1. Field of the Invention
This invention relates generally to tension leg platforms, and more particularly to an offshore tension leg platform (TLP) and a method for installing the same at a deepwater location.
2. Brief Description of the Prior Art
In the past, tension leg platforms (TLP's) have been designed to include the capability of performing well drilling operations. These platforms have supported the risers and well trees at the deck level. To meet these requirements, the platforms have to be large enough to provide buoyancy to support the payload of the well drilling equipment and the risers support system has to incorporate mechanisms which ensure the tension in each riser remains constant as the length of the risers varies due to movements of the hull caused by wave and environmental loadings. The size and complexity of these TLP's have resulted in costs which are too high for the commercial development of small offshore hydrocarbon reservoirs.
In the search for a means of producing from small reservoirs, various small TLP configurations have been proposed including TLP's without any drilling capability. All post-installation drilling activities have to be performed by a drilling vessel positioned alongside or over the TLP. The purpose of these platforms has been only to support the risers and position the well trees above the water surface. Production equipment and facilities have been provided on the separate drilling and support vessel, such as disclosed in U.S. Pat. No. 4,913,238 to Danazcko et al and U.S. Pat. No. 5,190,411 to Huete et al.
Other proposed small TLP's have included the provision of production facilities such as in U.S. Pat. No. 5,117,914 to Blandford. However, no capability for platform supported wells or well intervention operations has been included. In summary, all the proposed small TLP's have a requirement for a separate vessel in order that operations can be performed on the wells during the phase of producing from the reservoir.
The present invention provides all well intervention capabilities for production well completions, re-completions, workover and fluid injection operations. The technical complexity of the large TLP's is, however, avoided by minimizing the above water payload. Moreover the configuration of the platform hull reduces the loadings and thereby reduces the loadings in the anchored tendons. Furthermore, the placement of the well riser support mechanisms at a strategic position below the water surface simplifies the risers support mechanisms. Installation of the foundations, tendons and hull does not require a specialist deepwater cranevessel and equipment.
A tension leg platform of the prior art is described in U.S. Pat. No. 4,784,529 (Hunter), and Offshore Technology Conference Paper No. 6360 (1990) by Hunter et al. A drilling template seabed structure is generally provided to position and support the production wells, and this structure is, in turn, supported by piles. The template includes means by which the structure's elevation and inclination may be adjusted. A separate seabed foundation structure or structures are provided to anchor the platform. The foundation structure is secured to the seabed by steel tubular piles. These are too large to be installed by a conventional drilling vessel and equipment and in the past have been driven into the soil using specialist pile hammers, which in turn can only be operated by specially equipped installation vessels. The anchor piles are permanently connected to the foundation structure(s) by grout or mechanical means. The tendons are connected to the foundation structure(s) and not to the anchor piles. To sustain the imposed loadings, the tubular steel tendons are manufactured with high technology materials and assembly procedures. They have been either assembled ashore as complete units or sections have been jointed at site using specialist connectors and handling equipment. In both cases, special means, vessels and procedures must be used for the transportation and installation of tendons at site. To resist high fatigue loadings at the tendon connections special complex flex joint means are provided for freedom of rotation and to sustain cyclic loading variations. To support the high tendon pre-load tensions, the large deck, full well drilling facilities and the deck-supported well risers and trees, the hull is configured with large multiple (typically four) vertical corner columns which are joined by submerged pontoon elements (typically four). The resistance of the large columns to environmental loadings results in the need for higher tendon tensions to prevent pitching and rolling and heaving motions of the platform. Similarly, the larger waterplane area of the columns results in higher heave, pitch and roll induced loadings in the tendons.
In order to perform drilling operations on a well, the drilling rig is located directly above the seabed wellhead. On prior art tension leg platforms, this requirement has been fulfilled in one of two ways. The rig is movable and is positioned as desired on the platform deck. The movement of the large well drilling rig and the associated large variable load has to be accompanied by adjustment of the hull water ballast placement in order that tension forces in each of the tendons remain within limits. This requirement to adjust the volume and location of hull water ballast in turn means that the submerged portion of the hull must be large enough to provide corresponding excess volume and ballast position flexibility. The alternative means of positioning the rig without adjustment of the hull water ballast is to change the plan position of the hull with respect to the subsea wellheads. Means of positioning the hull which have been proposed are either by providing a platform mooring system and adjusting the lengths of the mooring lines to move the platform to the desired position, or by equipping the hull with thrusters which offset the hull to a desired horizontal position and thereafter maintains the position while operations are performed on a given well. Both alternatives require additional platform means and complexities which increase the weight which, in turn, increases the size of the hull, tendons and anchor foundations and increase the cost.
The drilling structure on the TLP deck is used to support not only drilling operations, but other well intervention operations such as well workovers. The drilling rig structure will be subjected to external loadings due not only to the wind loadings but also due to the horizontal acceleration of the platform. These lateral acceleration loadings can be large when significant tubular setback is placed inside the derrick of the drilling rig, and an extreme storm condition is experienced which produces large accelerations. The drilling structure therefore must withstand these dynamic loadings and this may require the rig to be a special design.
Prior art tension leg platforms support the well risers at the deck where the trees are located and the deck structure must therefore be strengthened to support the risers and transmit the payload to the columns of the hull, and the resulting increased weight of the deck results in an increase in the size of the supporting hull. The tendons, however, are connected at the optimum elevation at or close to the keel of the hull. When the platform is subjected to horizontal environmental loadings of wind, waves and current, the resulting forces push the hull sideways. However, since the hull is restrained vertically by the anchored tendons, the horizontal offset movement is accompanied by a corresponding downward vertical movement. This increase of the hull draft, or "platform setdown", causes the elevation of the tops of the well risers to be changed with respect to the elevation of the deck, and the tension loads in the riser must remain constant within defined limits. The changes in the riser lengths are caused by the difference in the distance between the bottom and the top points of connection of the tendons and the risers. In order to prevent undue tension variations of the risers these are suspended from the platform deck with variable stroke, constant tension devices. This equipment increases the platform complexity; and due to its weight, the size of the submerged portion of the hull must be increased to provide support for this payload.
A basic feature of the tension leg platform concept is that within prescribed limitations, the pretension loading in each tendon must be equal. Prior art platforms have incorporated complex means for adjusting the effective length and pretension of each tendon during the installation operations of connecting the tendons to the hull, as shown in U.S. Pat. Nos. 4,281,613 (Gunderson), 4,848,970 (Hunter, et.al.), 4,784,529 (Hunter) and 4,780,026 (Gunderson).
Prior art relating to TLP platform installation have relied on various special installation equipment and systems, such as crane vessels, which are expensive to operate in deep water. Also, prior TLP platform installations have used several different installation spreads for the various phases of installation of templates, foundations, tendons, hulls, and well drilling. This has required the expensive and time consuming mobilization and demobilization of various equipment.